< cpp‎ | algorithm
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
Sampling operations

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
Defined in header <algorithm>
template< class InputIt, class Size, class UnaryFunc >
InputIt for_each_n( InputIt first, Size n, UnaryFunc f );
(1) (since C++17)
(constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt, class Size, class UnaryFunc >
ForwardIt for_each_n( ExecutionPolicy&& policy,

                      ForwardIt first, Size n, UnaryFunc f );
(2) (since C++17)

Applies the given function object f to the result of dereferencing every iterator in the range [firstfirst + n). If f returns a result, the result is ignored.

1) f is applied in order starting from first.
If UnaryFunc is not MoveConstructible, the behavior is undefined.
2) f might not be applied in order. The algorithm is executed according to policy.
This overload participates in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)
If UnaryFunc is not CopyConstructible, the behavior is undefined.

If n >= 0 is not true, the behavior is undefined.

If the iterator type (InputIt/ForwardIt) is mutable, f may modify the elements of the range through the dereferenced iterator.

Unlike the rest of the parallel algorithms, for_each_n is not allowed to make copies of the elements in the sequence even if they are TriviallyCopyable.


[edit] Parameters

first - the beginning of the range to apply the function to
n - the number of elements to apply the function to
policy - the execution policy to use. See execution policy for details.
f - function object, to be applied to the result of dereferencing every iterator in the range [firstfirst + n)

The signature of the function should be equivalent to the following:

 void fun(const Type &a);

The signature does not need to have const &.
The type  Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to  Type.

Type requirements
InputIt must meet the requirements of LegacyInputIterator.
ForwardIt must meet the requirements of LegacyForwardIterator.
Size must be convertible to an integral type.

[edit] Return value

An iterator equal to first + n, or more formally, to std::advance(first, n).

[edit] Complexity

Exactly n applications of f.

[edit] Exceptions

The overload with a template parameter named ExecutionPolicy reports errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

See also the implementation in libstdc++, libc++ and MSVC stdlib.

template<class InputIt, class Size, class UnaryFunc>
InputIt for_each_n(InputIt first, Size n, UnaryFunc f)
    for (Size i = 0; i < n; ++first, (void) ++i)
    return first;

[edit] Example

#include <algorithm>
#include <iostream>
#include <vector>
void println(auto const& v)
    for (auto count{v.size()}; const auto& e : v)
        std::cout << e << (--count ? ", " : "\n");
int main()
    std::vector<int> vi{1, 2, 3, 4, 5};
    std::for_each_n(vi.begin(), 3, [](auto& n) { n *= 2; });


1, 2, 3, 4, 5
2, 4, 6, 4, 5

[edit] See also

applies a function to a range of elements, storing results in a destination range
(function template) [edit]
range-for loop(C++11) executes loop over range[edit]
applies a function to a range of elements
(function template) [edit]
applies a function object to the first N elements of a sequence